1. Field of the Invention
The present invention relates to an ignition device for an internal combustion engine, and more particularly, to an ignition device in which a distributor is omitted and a series gap utilized.
2. Description of the Related Art
A typical ignition device for a modern internal combustion engine includes an ignition coil, a distributor and spark plugs, but it is known to use a unit ignition system in which an ignition coil is provided for each cylinder of the engine and the distributor is omitted. In this system, however, it is necessary to provide a number of ignition coils corresponding to the number of engine cylinders. It is also known to use a combination of transistors and diodes to distribute the voltage surge from a single ignition coil to all of the spark plugs, and thus omit the distributor. Also, a capacitor discharge ignition (CDI) system is known, in which a capacitor is used together with a distributor and the ignition coil is omitted.
The CDI ignition system is mainly adopted for a two stroke multi-cylinder internal combustion engine, because the CDI system can produce the high voltage necessary for spark discharge at short time periods. For example, in a two stroke six-cylinder internal combustion engine, the distributing time interval must be no more than 1.7 milliseconds for each cylinder at 6,000 rpm. The CDI system can produce the necessary voltage in such a short time period, but an ignition coil system may need a time period of up to 5 milliseconds for one discharge.
The CDI ignition system, however, has a relatively small discharge energy, and thus suffers from a problem of misfires during a light load condition of the engine. In this aspect, it is preferable to use an ignition coil of the type which produces a high voltage surge when the primary current is shut off. However, it is difficult to use a distributor for a two stroke engine, since the time period for the ignition coil discharge distributed by the distributor, is not satisfactory, as above described. Therefore, it may become necessary to provide a unit ignition system having an ignition coil for each cylinder of the engine.
The two stroke engine also suffers from a problem of spark plug weak dischange.
Japanese Unexamined Patent Publication (Kokai) No. 52-156241 and Japanese Examined Utility Model Publication (Kokoku) No. 56-16799 disclose an ignition device having a series gap which is provided in series with the discharge gap of the spark plug. The series gap is advantageously used to supply an instantaneously rising high voltage, rather than a gradually rising voltage, to the spark plug, thereby preventing a weak discharge due to deposits on the electrode of the spark plug, which deposits cause an electrical leakage between the electrode and the body of the plug.
Therefore, in the case of a two stroke engine, the series gap system is preferably provided together with the unit ignition system. With this arrangement, however, the ignition coil must be made much larger in size because the series gap itself consumes discharge energy, and thus the ignition coil must have a large inductance. Accordingly, it becomes difficult to mount a plurality of ignition coils on the engine for each cylinder.